1. Field of the Invention
This invention relates to a semiconductor device and a manufacturing method therefor. More particularly, the invention relates to improvements in a semiconductor device and a manufacturing method therefor, the device including a buried impurity layer formed in a semiconductor substrate by using ion injection.
2. Description of the Background Art
It is known that, generally, a buried impurity layer is provided for an integrated circuit including a plurality of MOS transistors in order to prevent software errors due to alpha particles and to prevent latchups. It is also known that a buried impurity layer is provided for a bipolar transistor to act as a floating collector.
As shown in FIG. 9A which is a sectional view, a buried impurity layer 3a, usually, is formed by diffusing an element that determines the conductivity type, such as boron, phosphorus or arsenic, on a main surface of a semiconductor substrate 1, and superposing an epitaxial layer 1a in a thickness of several micrometers on the impurity layer 3a. The epitaxial layer 1a is covered with an insulating film 4 and isolation regions 5. A semiconductor device such as a MOS transistor (not shown) is formed on the epitaxial layer 1a in an area surrounded by the isolation regions 5. However, it is time-consuming and costly to form the buried impurity layer 3a by diffusion and to cause growth of the epitaxial layer 1a.
Thus, as shown in FIG. 9B, attempts have been made in recent years to utilize ion injection in forming a buried impurity layer in a short time and at a relatively low cost. More particularly, a buried impurity layer 3 is formed by ion injection, with a high energy in the range of several hundred keV to several MeV, of an element that determines the conductivity type, through an insulating film 4 into positions of a semiconductor substrate 1 at a depth of several micrometers. Then the substrate 1 is heat-treated in order to activate the buried impurity layer 3 and to eliminate primary crystal defects due to the ion injection.
During the heat treatment, the disappearance of the primary crystal defects in the buried impurity layer 3 due to the ion injection progresses inwardly from top and bottom of the impurity layer 3. However, secondary defects such as dislocations and stacking faults tend to remain in inner positions of the impurity layer 3. In the regions upwardly of the buried impurity layer 3 where the ions have passed, the primary defects such as vacancies tend to remain which retard recovery of crystallinity. Such residual defects can be a cause of increases in the leak current of the substrate.
Referring to FIG. 10, an example of methods for measuring the leak current of a semiconductor substrate 1 including a buried impurity layer 3 is illustrated. In FIG. 10, a p.sup.- substrate 1 includes a buried p.sup.+ impurity layer 3. An n.sup.+ impurity region 7 is formed on an upper surface of the p.sup.- substrate 1. The n.sup.+ impurity region 7 is connected through an ammeter 8 to a variable positive voltage source 9. The substrate 1 has a grounded bottom surface. In this way, the leak current may be measured by applying a reverse bias voltage to the semiconductor substrate 1.
Referring to FIG. 11A, an example of leak current of a substrate measured by the method of FIG. 10 is shown. In FIG. 11A, the substrate is injected with boron ions with an accelerating energy of 1.5 MeV in 1.times.10.sup.14 ions/cm.sup.2, and thereafter annealed in a nitrogenous atmosphere at 1000.degree. C. for one hour. The horizontal axis represents the reverse bias voltage (V), and the vertical axis the leak current (A). It will be seen that the leak current of the substrate increases markedly at the reverse bias voltage of about 3.5V and above, and that this substrate is unavailable for practical purposes. It is believed that this increase in the leak current is caused by the residual crystal defects due to the ion injection.
FIG. 11B shows, for purposes of comparison, a leak current of a semiconductor substrate including a buried impurity layer formed by diffusing boron and an epitaxial layer superposed thereon. In FIG. 11B, the leak current shows little increase at the reverse bias voltage up to about 17V since the semiconductor substrate includes no lattice defects due to ion injection.
As noted above, a buried impurity layer may be formed in a semiconductor substrate in a short time and at low cost by utilizing high energy ion injection. However, a substrate including a buried impurity layer formed in this way is not fit for practical use because of the great leak current.